You're right, negative number won't work, and you can't separate a flange from the vessel. However, I have a solution for you. The first thing you need to do is to draw an AutoCAD line at the point where the nozzle would be welded to the outside. Next is to insert a flange, the size and spec needed, floating in space near the vessel with the flange face pointing toward the vessel centerline IF the nozzle was in the tank. If you placed it now, the OD of the weldneck would show through the vessel. Move the ACAD line 1/2 the diameter of the flange pipe size toward the center of the vessel. Now move the floating flange from the node of the weldneck to the end of the ACAD line. Now it's inside, but you're not done. Change the view to wireframe to see the internal flange. Continue routing from the internal flange with a mating flange and a short pipe segment. Here's the trick. Go to Equipment/Attach Equipment, select the vessel, and then the first internal flange. Click on the vessel and the new flange is attached to the inside. Last step is to erase the temporary ACAD line. Now you can continue routing internally from the pipe segment. Good luck.